1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich an organic compound-containing layer, and to a method of manufacturing the light emitting device. In this specification, the term light emitting device includes an image display device, a light emitting device and a light source (including illuminating device). Also, the following modules are included in the definition of the light emitting device: a module obtained by attaching to a light emitting element a connector such as an FPC (flexible printed circuit; terminal portion), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or the TCP; and a module in which an IC (integrated circuit) is directly mounted to a light emitting element by the COG (chip on glass) system.
In addition, a semiconductor device referred to in the specification of the present application indicates devices in general making use of semiconductor characteristics to be able to function, and electro-optic devices, semiconductor circuits, and electronic apparatuses are all included in the semiconductor device.
2. Description of the Related Art
Light emitting elements, which employ organic compounds as light emitting members and are characterized by their thinness and light weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among display devices, ones having light emitting elements arranged to form a matrix shape are considered to be particularly superior to the conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich an layer containing an organic compound, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the organic compound-containing layer to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Known as excitation states are singlet excitation and triplet excitation, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having light emitting elements arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. However, if the pixel density is increased, active matrix light emitting devices in which each pixel (or each dot) has a switch are considered as advantageous because they can be driven with low voltage.
Organic compounds for forming an layer containing an organic compound (strictly speaking, light emitting layer), which is the center of a light emitting element, are classified into low molecular weight materials and polymeric (polymer) materials. Both types of materials are being studied but polymeric materials are the ones that are attracting attention because they are easier to handle and have higher heat resistance than low molecular weight materials.
The conventional light emitting device has the structure comprising a light emitting element in which an electrode electrically connected with a TFT on a substrate is formed as an anode, an organic compound layer is formed thereon, and a cathode is formed thereon. And light generated at the organic compound layer can be observed at the TFT side through the anode that is a transparent electrode. There has been a problem in the structure that an opening ratio is restricted depending on an arrangement of TFTs and wirings in a pixel portion when definition is to be improved.
Alternatively, a structure (hereafter is referred as top emission structure) is given in which an anode is formed as an electrode, an organic compound-containing layer is formed on the anode, and a cathode serving as a transparent electrode is formed on the layer containing organic.
In an top emission structure, there has been a problem in that a wiring, a TFT, or the like is provided in regions other than a light emitting region so that light reflected by the wiring reaches eyes of an observer.